1. Field of the Invention
The invention relates to fluid delivery systems. More particularly, the present invention relates to detecting air and other agents in a fluid delivery system infusing fluid to a patient.
2. Description of Related Art
There are a variety of situations where fluid is infused to a patient. Applications of fluid delivery systems include (but are by no means limited to) intravenous infusion, intra-arterial infusion, infusion of enteral solutions, infusion of medication to the epidural space, and diagnostic infusion to determine vascular characteristics of the arterial, urinary, lymphatic, or cerebrospinal systems.
Fluid delivery systems for infusing fluid to a patient typically include a supply of the fluid to be administered, an infusion needle or cannula, an administration set connecting the fluid supply to the cannula, and a flow control device, such as a positive displacement infusion pump. The administration set typically comprises a length of flexible tubing. The cannula is mounted at the distal end of the flexible tubing for insertion into a patient's blood vessel or other body location to deliver the fluid infusate to the patient.
During an infusion procedure, various agents, the most typical of which is air, can be introduced into the fluid delivery system by a number of events, including the fluid supply becoming drained of fluid. Because introducing excessive air into the patient's blood system may create complications, it is desirable to detect the introduction of air into the fluid delivery system before substantial amounts of air are introduced into the patient. When substantial amounts of air are detected in the fluid delivery system, fluid delivery can be terminated until a health care provider can correct the underlying problem, such as by refilling or replacing the fluid supply.
Sometimes, a temporary event, such as the accumulation of small quantities of air from outgassing of air suspended in the solution, may cause a very few small air bubbles to enter the system. Where the amount of air is quite small, the patient may be able to safely absorb the small air amounts, so that stopping operation of the pump is unnecessary. Thus, it is desirable to not only detect the air in the fluid delivery system, but also to evaluate the amount of air present.
One technique for determining the amount of air in a fluid delivery system, such as a length of intravenous tubing, is through the use of sensors such as light or ultrasonic sensors. In such a technique, electromagnetic energy, such as light, or sound energy, such as an ultrasonic pulse, is passed through the intravenous tubing, and the sensor monitors variations in the received energy. Because air generally transmits light and/or sound energy in a different fashion than do intravenous fluid solutions, due to different transmission properties such as absorption and/or refractivity, monitoring variations in the light's or sound's ability to pass through the solution can give a generally accurate determination that air exists in the fluid line.
A more difficult problem is determining just how much air is in the fluid line, and how much will be delivered to the patient. For example, at a particular point in time, a sensor looking at just a very short section of the tubing may see only air in the line, with no intravenous fluid solution present. This may be the result of the fluid supply being entirely empty, in which case the fluid delivery system should be shut down. However, a single small air bubble may also cause the same sensor reading, and shutting down the fluid delivery system on account of a single air bubble may be inappropriate.
A small amount of air may be of no consequence where no significant amounts of air are in the delivery system either upstream or downstream of the sensor section. Where the small amount of air is part of a continuous stream of small air bubbles in the tubing, however, the sum of the small bubbles may amount to a significant amount of air, so that the fluid supply system should be shut off pending correction of the underlying problem.
A method of accounting for the limitations of monitoring just a short section of the tubing is to install several sensors along the length of the tubing, thereby monitoring a much longer section of tubing. The addition of multiple sensors and their associated electronics can, however, substantially add to the cost and complexity of the fluid delivery system. Moreover, such use of multiple sensors may still not accurately determine the amount of air in the line over long periods of time or as large volumes of fluid pass therethrough.
A further method is to keep a running total of the air that passes through the tubing section. When the total air reaches a certain threshold, the fluid delivery system can be shut down to await correction of the underlying problem by appropriate personnel. Such a simple running total may not, however, adequately reflect the actual ability of the patient's system to safely absorb air.
Hence, those skilled in the art have recognized a need for a fluid delivery monitoring system that can detect air in the fluid, but that can also take into account the total air over a period of time or in a volume of fluid, as well as to account for other factors, such as the ability of the patient's body to safely absorb some air during fluid volume infusion. The present invention satisfies these needs and others.